Acoustic device for sound pick up



Jull-L n woLFF ET AL 1,814,357

ACOUSTIC DEVICE FOR SOUN) PICK-UP Filed June 27. 1929 figg. 4

mem/mer INVENTORS HARRY F. oLsoN BY mvrw FF ATTORN EY Patented July 14, 1931 UNITED STATES IRVING WOLFF, OF NEW YORK,

AND HARRY vIF. OLSON, 0F BRONX, NEW YORK, AS-

SIGNORS TO RADIO CORPORATION OF AMERICA, A CORPORATION 0F DELAWARE ACOUSTIC DEVICE IE'OR- SOUND PICK UP Application led June 27, 1929. Serial 17o. 374,174.

This invention relates to acoustic devices. More particularly it relates to acoustic devices for sound pickup such as are ,used in radio broadcasting, and in the talking moving picture field.

Heretofore, the usual method of picking up sound has been to place the microphone or pickup device in the immediate vicinity of the source of sound. For example, in radio broadcasting and the like the microphone is placed approximately two feet from the speaker and directly in front of him. This nearness of the microphone or pickup device to the source of sound is not objectionable in everyday radio broadcasting or in some of the other ordinary usages of a pickup device but in some cases, such as in some particular instances of radio broadcasting and in the talking moving picture field, it has been found very objectionable.

For example, in making talking movies the microphone cannot be placed near the speaker or other source of sound for the reason that the microphone would then be in the scene. In order to keep the'microphone out of the scene it is necessar to place it at a point at least twelve or fifteen feet from the speaker and in many, cases the distance between the speaker and microhone is much greater than that. This has een found to be objectionable for the reason that the intensity of the sound reaching the microphone is diminished to such an extent that amplification is necessary to bring the electric volume up to the desired amount. Upon increasing thev amplification of the amplifier associated with the microphone, the ratio of. the ground noises to the desired sound is increased. Furthermore, when the microphone is placed at a distance from the speaker or source of sound the room resonances are picked up by the microphone to such a degree as to become very objectionable. Finally, the sound waves reflected by various parts of the scenery and room reach the microphone out of phase with the unrellected sound waves and this results in a mushed or distorted sound. v

Various arrangements have been tried for overcoming these ob'ectionable features such as the use of horns or collecting and intensifying the sound. The ordinary horn is found objectionable, however, when used as a sound intensifier in connection with a microphone or pick-up device for the reason that the amplification of the high frequencies is very small when compared to the amplification of the low frequencies. This is probably due to the destructive interference of the waves reflected from the walls of the hornysuch reflection beginning when the wave length of the impinging sound wave becomes smaller than the size of the horn and increasing with the decrease in wave length.

A second objection to the use of horns as sound intensifiers is found in the resonance peaks which occur at frequencies corresponding to the natural period or periods of the horn. At these frequencies the sound picked up by the microphone is out of all proportion to the sound emanating from the It is the object of this invention to provide a method of and lapparatus for sound pickup which will overcome or minimize these difficulties. More particularly it is the object of this invention to provide a method of and apparatus for picking up sound from a distant source in a manner such that there will be a relatively low percentage of disturbing or ground noises to the desirable sound.

Another object of this invention is to pro- Vide a device for the purpose of picking up sound from a distant source, which has directional properties whereby the objectionable efects of room resonances and reflected sound waves are reduced or eliminated.

Still another object of this invention is to provide a deviceV for intensifying sound waves from a distant source, which has the desirable ropertiesof both a horn and a re- Hector and) which at the same time minimizes or eliminates the objectionable properties of such devices.

A still further object of this invention is to provide a combined reflecting and intensifying device with means for mimizing theI objectionable effects of the, natural resonance soid it will be found that sound can be picked up at a relatively great distance from the source without a material increase inthe ratio of ground noises to desirable sound. It will also be found that such a device will have the desired directional properties for eliminating'or reducing to a minimum the objectionable effects of room resonances and reflected sound waves. L

The device functions at low frequencies to collect and intensi the sound waves in the same manner as W en an ordinary horn is used. At high frequencies it functions as a reflector and rovides a plurality of equal length paths between the source of sound and the microphone. As a result-the sound input to the micro hone is increased at both the low and high requencies.

In order to overcome the objectionable resonance peaks which occur at -the natural period or periods of the device, one or more sound resonators are associated with the device. By properly designing and locating the sound resonators they can be made to absorb just enough sound at the desired freqency or frequencies to overcome or balance out the increased sound at that frequency or frequencies( For a more complete understanding of the invention reference is had to the followingl specification which should be read in connectlpln with the accompanying drawings in W ich Figure 1 is a diagrammatic illustration of the invention;

Figure 2 is a sectional view of an approved form of the device diagrammatically illustrated in Figure 1;

Figure 3 is a front view of a slightly modified form of the device illustrated by Figure 2;

Figure 4 is a rear view of the device' illustratedby Figure 3; and

Figure 5 is a characteristic curve for the 4levice illustrating the effect of the resonaors. i

Referring more particularly'to the drawings, Fi/,gure 1 is a diagrammatic illustration oft plie'd to picking u sound from a relatively distant jsource. n thisv illustration the pinge against -the reflector.

e principles of my invention as 'ap source of sound which may, for example, be the speaker ina talking movie scene, 1s 1ndicated by reference numeral 10 and the m1- crophone is indicated byreference numeral 11. The member for reflecting and intensifying the sound waves fromthe source 10 1s indicated at 12. This member is designed so that its inner surface is an elli soid of revolution one focus of the ellipsoid being at the source 10 and the other focus being at the microphone 11.

. en sound waves are emittedfrom the source 10 the waves which have a short wave ing in the member 12, impinge against the inner surface of member 12 and are reflected against the microphone located at 11. Due to the characteristic of the elliptical curve the paths travelled by these sound waves are the same length regardless of where they im` For example, several paths are indicated at 13, 14 and 15. It will be notedpthat sound waves following these diagrammatic pathsare reflected by the member 12 at points 16, 17 and 18 to the microphone 11. In each case the total distance is the same, that is, 10, 16+`16,. 11=10, 17-1-17, 11:10, 18-1-18, 11. As a result of this arrangement the high frequency sound waves are reflected s o that they arrive at the focus point in phase regardless of where they impinge against the member 12. This prevents interference tending to distort the sound picked up by the microphone 11. It

`will be noted that sound waves from the source 10 may also follow the direct path between the source of sound and the microphone 11 but the proportion of sound waves affecting the microphone in this manner is so small compared to the pro ortion of sound waves reflected by the mem er 12 that their effect can be neglected.

The low frequency waves are not reflected in the same manner due to the length of the sound waves but they. are collected and intensified by the member 12 flmctioning as an ordinary horn.

In actual practice the distance between the microphone and the source of sound may vary rom" twelve to thirty feet and in some cases the microphone has been placed at even greater distances from the source of sound with good results. Theoretically a different ellipsoid should be used for each distance between the source and the microphone but in actual practice it has been found that good results are obtained leven when the source of is composed o f the 4member 12, themicrophone 11, which has its diaphragm 19 located at the 'focus point of the ellipsoid, and one lor more resonators 20,21. As shown by the` length compared to the diameter of the open- 1n .any approved manner.

`.ner surface to The member 12 may be made of any suitable material. The only requirement is that the finished member should have a relatively good reflecting surface. It has been found that sheet metal such as copper is readily shaped and that the resultant metallic member functions satisfactorily. The member could also be made of wood or moulded from suitable plastic material. In the latter cases it would probably be advisable to apply a suitable substance such as shellac to the inprevent the sound waves from being absorbed. p

Sound reflecting and intensifying members as described by the foregoing specification, usually have one or more natural reso nant periods within the audible range. In order to overcome the resonance peaks corresponding to these natural periods one or more resonators are operatively lassociated with the member 12 in a manner such that they absorb the excessive sound at these frequencies. Figure 2 illustrates the application of two such resonators20 and 21 to the memberl2.

The resonators consist of hollow chambers 23, usually cylindrical, which are connected to the interior vof the member 12 through relatively short neck-like portions 24. Each resonator is provided with a plug 25 which is adjustably positioned in the cylindrical chamber. The frequency at which the resonator absorbs energy may be varied by adjusting the plug but it is only necessary to tune the resonators initially as the natural period of the member 12 doesl not change. It has been found that there are usually two ob'ectionable resonance peaks occurring in the ylower frequencies so one or more resonators are preferably used for each peak. In the modification illustrated by Figure 2 only two resonators are used and as the resonators arev adapted to absorb energy at different frequencies one resonator is somewhat larfrer than the other. The resonators are pre erably made of metal with either a wooden or other suitable plug for tuning purposes.

Figures 3 and 4 are front and rear views respectively of the modification illustrated by Figure 2 with the exception that two res onators are used for each resonance peak making a total of four resonators. When two resonators are used for each peak they are usually tuned to slightly different frequencies so that absorption occurs over a fairly broad band and not over just a narrow band. The resonators may be connected to the member 12 at any desired points but for convenience and in order that they may offer a minimum amount of interference to the reflected waves they are located at the rear of the member 12 immediately adjacent the casing of the microphone 11. The neck 24 of each resonator is allowed to protrude a short distance into the member 12 in order to obtain the best results.

If the microphone is to be located at a relatively great distance from the. source of sound, the inner surface of the member 12 may be given the shape of a paraboloid of revolution. rlhe microphone in such a case, is located at the'focus of the paraboloid.

If an increase in the low frequency response is desired it can be obtained by increasing the length of the member 12. For simplicity of construction the increased part may bemade in the form of a frustium of a ycone-without materially decreasing the reflecting action of the member. The reason for this is that the surface of an ellipsoid of revolution approximates a conical surface as the surface recedes from the focus.

Figure 5 illustrates a characteristic curve for the complete device. The heavy curve indicates the characteristic of the device with the resonators 2() and 21 attached to the member 12 and tuned to the proper frequencies. The dotted curve illustrates the two peaks which are removed by the absorbing action of the resonators. As shown by the curve the pressure for the higher frequencies is a little greater than the pressure for the lower frequencies. This is just the opposite from the pressure curve .of an ordinary microphone or pickup device when used at long distances. j

It is to be understood that the present invention is applicable in various other fields than those to which reference has been made and also that various modifications may be made to the reflecting intensifying member without departing from the spirit of the invention. It is accordingly intended that the invention is not to be limited by the illustration and description of the preferred modification but only by the scope ofthe appended claims.

IVe claim:

1. The method of transferring energy from a source of sound to a microphone which consists in providing a plurality of equal length paths for the sound waves between the source of sound and the microphone, and in absorbing excess energy thus transferred to the microphone. 2. The method of transferring energy from a relatively distant source of sound to a microphone which consists in reflecting the sound waves from the source to the microphone through a plurality of equal length paths from a reflector partially surrounding the microphone and extending only a relatively short distance therefrom.

.3. The method of supplying Venergy to a microphone from a relatively distant source of sound which consists in reflecting sound Waves from thesource of sound to the microphone at a plurality of points located only in the immediate vicinity of the microphone and at different distances from the microphone, said points being arranged so that the sound Waves arrive at the microphone in phase.

jl. rllhe method of supplying energy to a microphone from a relatively distant source of sound which consists in reflecting sound Waves from the source of sound to the microphone through a plurality of different angles from a' reflector vpositioned adjacent only the microphone in a manner such that the sound waves arrive at the microphone in phase.

5. The method of transferring energy from asource of sound to a microphone which consists in roviding more thantwo equal length sound) Wave paths between the source of sound and the microphone, each of said paths including a point falling on an ellipse one focus point of which is the 'source of sound and the other focus point of which is the microphone.

6. -The method of supplying energy from a relativelydistant source of sound to a microphone which consists in reflecting the high frequency sound Waves from the source to the microphone through a plurality of equal length paths and in collecting and intensifying the low frequency sound Waves without reflection.

7. The method of transferring energy from a source of sound to a microphone which consists in collecting energyfrom said source of sound in a manner such that objectionable resonance peaks are obtained, absorbing energy Aat the frequencies of th'e resonance peaks, and supplying the remain'- ing energy to the microphone.

8. The method of transferring energy fromv a source of sound to a microphone which consists in' collecting energy from said source in a manner such that an objectionable resonance peak is obtained, absorbing just sufficient energy at Vthe frequency of the resonance'peak to substantially eliminate the resonance peak, and supplying the collected energy without the objectionable resonance peak to the microphone.

9. The combination of a source of sound, a microphone, and means positioned adj acent said microphone only for reflecting sound Waves from the source of sound to the microphone through a pluralityv of e ual length paths, said means being forme. so that the points of reflection do not lie in a single lane. Y

10. he combination of means constituting a source of sound, means constituting a microphone, and means positioned in the vicinity of only one of the aforementioned means for reflecting sound Waves from the source of sound to the microphone from a plurality of points not arranged in the same plane, in a manner such that the soundwaves arrive at the microphone in phase.

11. The combination offa source of sound, a microphone, and means for reflecting sound waves from the source of sound to the microphone through a plurality of equal length paths, said means being such that the points of reflection are located at varying distances from the micro hone but in th immediate vicinity thereo 12. The combination of a member whose inner surface has substantially the form of an ellipsoid yof revolution, means located at one focus of said ellipsoid of revolution for changing sound vgaves into electrical vibrations, and a source of sound located at the other focus of said ellipsoid of revolution.

13. An acoustic device comprising a member Whose inner surface has the form of an ellipsoid of revolution, means located at the focus of said ellipsoid of revolution for the interconversion of sound and electrical energy, and sound absorbing means associated with said member.

14. An acoustic device comprising a microphone, a sound Wave intensifying member operatively associated with said microphone, and means -associated with said intensifying member for absorbing energy at a predetermined frequency.

15. An acoustic device comprising a microphone, a sound intensifying member which has a natural period within the audi.- ble range, said microphone being positioned to receive energy from said sound intensifying member, and means for absorbing energy at the frequenc of said natural period.

.16. An acoustic evice comprising a microphone, a horn-like member for intensifying the sound to be received by said microphone, said horn-like member having a natural period Within the audible frequency range whereb a plurality of resonance peaks are obtained, and means for absorbing energy only at the frequencies of said resonance peaks.

17. The combination of a source of sound, a micro hone, a member for reflecting sound waves rom the source of sound to the microphone through a lurality of. equal length paths, said member eing shaped so that the points of reflection are located at varying distances from the microphone, and means for preventing excess ener y at known frequencies from affecting sai microphone.

18. An acoustic device comprising a micro hone, a sound intensifying member for sai microphone, said sound intensifying member having a natural period Within the audible range, and means connected to the interior of said sound intensifying member for preventing said microphone from being affected by an excess of energy at the fre quency of said natural period.

1 9. The 'combination of a source of energy, means ada ted to be affected by the energy from sai source, means associated with the first mentioned means for collecting energy from said source, said last mentioned means also functioning to generate an excess of energy at at least one frequency within the audible range, and means for reventin the excess of ener generate by the col ecting means from a ecting said'rst mentioned means.

20. An acoustic device comprising a microphone, a horn-like member for intensifying the sound to be received by said microphone, said horn-like member having at least one natural period within the audible frequency range whereby a plurality of resonance peaks are obtained, and means for absorbingy energy at the frequencies of said resonance peaks, said means comprising a plurality of sound resonators tuned to `substantially the frequencies at which said resonance peaks occur. v

21. An acoustic device for sound pickup comprising a member whose inner surface has the form of an ellipsoid of revolution, a microphone located at the focus of said ellipsoid and a plurality of resonators for absorbing excess energy from saidmember.

22.. An acousticy device for sound pickup comprising a member whose inner surface has the form of an ellipsoid of revolution, a

- microphone located at the focus of said ellipsoid and a plurality of -resonators for absorbing excess energy from said member, said resonators being tuned to at least two different frequencies.

23. A11 acoustic device Lfor sound pickup comprising a member whose inner surface has lthe form of an ellipsoid of revolution, a micro hone located at the focus of said ellipsoi and a plurality of resonators for absorbing excess energy from said member, said resonators being connected to the interior of said member adjacent said microphone.

24. An acoustic device for sound pickup comprising a member whose inner surface has the form of an ellipsoid of revolution, a microphone having its diaphragm perpendicular 'to the axisof revolution at the focus of said ellipsoid, said microphone having a casing substantially cutting off that part of the ellipsoid in back of the dia hragm, and a plurality of resonators for a sorbing excess energy from said member said resona tors being connected to the interior of said member at points near the front of said diaphra gm IRVING WOLFF.

HARRY F. OLSON. 

